Doppler ultrasound imaging systems are used to map blood flow in target regions of the human body. Ultrasonic energy is transmitted into a target region by an ultrasonic transducer. A portion of the ultrasonic energy is reflected back to the transducer. Moving cells, typically red blood cells, which have a velocity component toward or away from the transducer produce a frequency shift in the signal that is received by the transducer in accordance with the well-known Doppler principle. Red blood cells moving toward the transducer shift the ultrasound carrier higher in frequency, and cells moving away from the transducer shift the carrier lower in frequency.
The reflected signals received by the transducer are detected by a quadrature detector to provide in-phase and quadrature components of the received signal. The in-phase and quadrature signals are filtered, and a discrete Fourier transform is performed, using either digital techniques such as the fast Fourier transform or analog techniques such as the chirp-Z transform. The output of the discrete Fourier transform is a spectrum of upper sideband frequencies, or frequencies above the ultrasound carrier frequency, and a spectrum of lower sideband frequencies, or frequencies below the ultrasound carrier frequency. The Doppler equation is used to convert the sideband frequencies to velocities. The upper and lower spectra are usually nonsymmetric, since they represent forward and reverse velocities of red blood cells in the target region.
The single sideband quadrature detection technique described above uses parallel detection channels to extract the upper and lower sideband frequency information from the received signal. Each channel contains both the forward and reverse flow signals. The signals are differentiated by the phase relationships between the parallel channels.
Mirroring refers to a phenomenon in Doppler ultrasound imaging systems wherein forward flow toward the transducer appears as a reverse flow away from the transducer, and vice versa. Thus, blood flow at a particular velocity appears in the spectrum of the Doppler system as both a forward and a reverse velocity.
It is known that mirroring occurs as a result of mismatches between the in-phase and quadrature channels of the signal processing circuitry. More specifically, the quadrature detector may introduce an undesired phase shift between in-phase and quadrature signals. The filters used in the in-phase and quadrature channels may have magnitude and phase mismatches. Finally, the analog-to digital converters which digitize the in-phase and quadrature signals prior to performing the discrete Fourier transform may be mismatched. Mirroring is eliminated entirely only when the in-phase and quadrature channels are perfectly matched. The problem is complicated by the fact that the magnitude and phase of the mismatches vary with frequency so that the amount of mirroring varies as a function of frequency. In the past, mirroring has been reduced by using precision components in the in-phase and quadrature channels and/or by manually adjusting components to eliminate mismatch. However, since the magnitude and phase of the mismatch vary with frequency, mirroring may be eliminated at one velocity but may be made worse at other velocities. Furthermore, the use of precision components or manual adjustment of components adds to the cost of the system.
Signal processing circuitry which utilizes single sideband detection followed by discrete Fourier transformation is used in other applications. Examples of such applications include spectrum analyzers, network analyzers, Doppler radar systems and sonar systems. Mirroring can occur in such systems.
It is a general object of the present invention to provide improved methods and apparatus for medical Doppler ultrasound imaging.
It is another object of the present invention to provide method and apparatus for reducing or eliminating mirroring in medical Doppler ultrasound imaging systems.
It is a further object of the present invention to provide methods and apparatus for highly accurate spectral processing of signals.
It is yet another object of the present invention to provide Doppler ultrasound imaging systems which are low in cost and easy to manufacture.
It is a further object of the present invention to reduce or eliminate mirroring in signal processing circuitry which employs single sideband detection and discrete Fourier transformation.